The energy required to break one mole of Cl – Cl  bonds in $C_{12}$ is 242 kJ  $mo{l}^{-1}$. The longest wavelength of light capable of breaking a single Cl – Cl bond is $(c=3\times {10}^{8}m{s}^{-1}and{N}_A=6.02\times {10}^{23}mo{l}^{-1})$